During a very interesting and news-filled interview by Sandy Munro, Tesla's Elon Musk revealed that the upcoming refreshed Tesla Model S/Model X will be equipped with a lithium-ion 12V auxiliary battery.
The company has started a process of transition from lead-acid 12V batteries to lithium-ion 12V batteries, from its top of the line models by the way of a broader upgrade.
The reason for the change to lithium-ion is to match the calendar life of the main traction battery pack and offer "way" higher capacity. At this point, we don't know yet whether the new 12V battery is smaller (it could be because of the multiple-times higher energy density) or if the form factor is the same. We guess it will be lighter though.
Other unknown things are whether Tesla has built the 12V battery on its own (we guess so), cell type (the old 18650, 2170 or 4680?) and chemistry (it may be different than the cells in the main pack to cover wider temperature range).
In general, the lithium-ion battery might be a few times more expensive than lead-acid, but the lack of necessity to replace it every several years and the potential higher reliability might actually lower the total cost of ownership.
It's a very interesting topic also because this is only the first step. Besides the switch to lithium-ion batteries, Elon Musk clearly expressed the opinion that 48V would be better and crucial to use the same wires for power and data.
In other words, more capable, more reliable and lighter electrical systems are in the pipeline for this decade.
For those who are interested in Model 3, Model Y or other Tesla cars, we would like to add that there is no info about lithium-ion 12V batteries for them just yet.